Electrical Properties of ZnO Thin Films Deposited by Pulsed Laser Deposition
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Electrical Properties of ZnO Thin Films Deposited by Pulsed Laser Deposition. S. P. Heluani1, G. Simonelli1, M. Villafuerte1, G. Juarez1, A. Tirpak2, G. Braunstein2, F. Vignolo3 1
Laboratorio de Física del Sólido, Departamento de Física, Fac. de Ciencias Exactas y Tecnología, Universidad Nacional de Tucumán, Argentina. 2 Department of Physics University of Central Florida, Orlando, FL, 32816, USA. 3 Laboratorio de Ablación Láser, Fac. de Ingeniería, Universidad de Buenos Aires, Argentina ABSTRACT Structural and electronic transport properties of polycrystalline ZnO thin films, prepared by pulsed laser deposition, have been investigated. The films were deposited on glass and Si3N4/Si substrates using O2 and N2 atmospheres. X-ray analysis revealed preferential c-axis orientation perpendicular to the sample substrate. Films deposited under relatively high O2 pressure were highly resistive. However, the conductivity σ increased while the films were irradiated with ultraviolet light, showing an Arrhenius (ln σ ∝ T-1) dependence as a function of temperature. The ZnO film deposited in N2 atmosphere exhibited at room temperature a resistivity ∼ 1 Ω cm, and a sheet carrier concentration ∼ 5 1012 cm-2. The variation of the conductivity with temperature, in the range 60 – 150 K, follows a ln σ ∝ T-1/4 dependence characteristic of variable range hopping. An analysis of the experimental results of conductivity as a function of temperature, in terms of possible doping effects, as well as conduction mechanisms is presented. INTRODUCTION Zinc oxide is not only studied as an important component for next generation short-wavelength optoelectronic devices [1-5], but also as a potential spintronics material [6]. It is a wide band gap semiconductor that can be made n-type via intrinsic or extrinsic doping. Heavily doped ZnO behaves as a degenerate semiconductor with metallic-like conduction. In recent years, several authors have explored experimentally and theoretically the n-type and p-type doping and codoping in ZnO and its compounds. It is known that ZnO behaves as n-type semiconductor when it is grown in oxygen deficient or Zn rich atmospheres. p-type doping is more difficult, but the possibility to obtain acceptor levels in ZnO by doping with nitrogen, phosphorus and arsenic has been reported [7-10]. The difficulties encountered when trying to obtain good, reliable, p-type material may be related to the fact that the physics of defects in ZnO is quite complex, and to a significant extent unknown. In addition, the understanding of the mechanisms of electrical conduction in this material are still incomplete and challenging [9, 11]. We have started a systematic study of the electronic transport properties of ZnO thin films and in this work we report on the electrical characterization of thin films of ZnO grown by Pulsed Laser Deposition (PLD) under different atmospheres of O2 and N2. The effect of the growing atmosphere on the electrical transport properties, studied by measuring the resistivity as a function of temperature
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